[TZO-16-2020] - F-SECURE Generic Malformed Container bypass (GZIP) Feb 24 2020 09:38AM
Thierry Zoller (thierry zoller lu)

From the low-hanging-fruit-department
F-SECURE Generic Malformed Container bypass (GZIP)

Ref : [TZO-16-2020] - F-SECURE Generic Malformed Container
bypass (GZIP)
Vendor : F-SECURE
Status : Patched
CVE : CVE-2020-9342
Blog :
Vulnerability Dislosure Policy: https://caravelahq.com/b/policy/20949

Affected Products
F-Secure Email and Server Security
F-Secure Internet GateKeeper

Linux below 17.0.605.474

I. Background
Quote: "Unprecedented challenges threaten to undermine the very survival
of society. Only unprecedented innovation can prevent irreversible
disasters. This is only possible if we
trust the technology that can bring us together.

From our humble beginnings more than thirty years ago, F-Secure has
grown into a reliable cyber security leader, earning the trust of
organizations and people around the world."

II. Description
The parsing engine supports the GZIP Archive. The parsing engine can be
bypassed by manipulating a GZIP archive (Compression Method). This way
the User can extract the file but the AV Engine cannot giving the file a
clean pass.

III. Impact
Impacts depends on the contextual use of the product and engine within
the organisation of a customer. Gateway Products (Email, HTTP Proxy etc)
may allow the file through unscanned and give it a clean bill of health.
Server side AV software will not be able to discover
any code or sample contained within this ISO file and it will not raise
suspicion even if you know exactly what you are looking for (Which is
for example great to hide your implants
or Exfiltration/Pivot Server).

There is a lot more to be said about this bug class, so rather than bore
you with it in this advisory I provide a link to my 2009 blog post

IV. Patch / Advisory
- For the unix version (IGK) the fix was released on 20th January with
library version 17.0.605.474
- For Windows the version was already out in December 2019. There were
also avira's fixes as well.

"We can conclude that January 20th date, since that is when the final
fixes went out to production."

V. Disclosure timeline

We would like to thank F-Secure for their customer oriented way of
handling this vulnerability.

- NOV 14 2019
Initiated Vulnerability coordination

- FEB 05 2020
F-Secure notifies me that they have patched the flaw.

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